Crystalline silicon solar cells produced using a crystalline silicon substrate have high photoelectric conversion efficiency, and have already been widely put into practical use as solar photovoltaic power generation systems. Among them, a crystalline silicon solar cell in which a silicon-based thin-film having a gap different from that of single-crystalline silicon is formed on a surface of a single-crystalline silicon substrate to form a semiconductor junction is called a heterojunction solar cell. Among heterojunction solar cells, a solar cell in which a thin intrinsic silicon-based thin-film layer is interposed between a conductive silicon-based thin-film layer as an emitter or a base and a surface of a crystalline silicon substrate is one of forms of crystalline silicon solar cells having the highest conversion efficiency. It is known that when a thin intrinsic silicon-based thin-film layer is formed between a surface of a crystalline silicon substrate and a conductive silicon-based thin-film layer, surface defects on the crystalline silicon substrate are terminated to improve conversion efficiency.
In heterojunction solar cells, similarly to other solar cells, metal electrodes (collecting electrode on the light-receiving side, and back side metal electrode on the back side) are provided, and carriers generated in crystalline silicon are collected. A transparent electrode formed of a transparent conductive oxide (TCO) etc. is inserted between the conductive silicon-based thin-film layer and the metal electrode.
Thus, in a heterojunction solar cell, a conductive silicon-based thin-film layer formed on a surface of a crystalline silicon substrate and a metal electrode are not in direct contact with each other, and therefore formation of a recombination center due to diffusion of metal elements from the metal electrode is prevented to maintain quality of passivation (termination of surface defects) by an intrinsic silicon-based thin-film.
One of the problems of crystalline silicon solar cells is that the cost of metal electrode materials is high. Particularly in heterojunction solar cells, an amorphous conductive silicon-based thin-film layer is used, and therefore the allowable temperature in junction is low, so that conditions for sintering metal electrode materials are constrained. For securing sufficient conductivity, a large amount of metal electrode materials are needed, leading to a further increase in cost.
Since the influence of a shadowing loss is negligible on the back side, an attempt has been made to increase the area of a back side metal electrode by, for example, forming the back side metal electrode over the entire back surface. For example, when a black colored back sheet which is favored in terms of an external appearance is used as a back sheet for a solar cell module, light arriving at the back side cannot be made to reenter a solar cell by reflection of the light at the back sheet. Accordingly, it is effective to form a back side metal electrode over the entire surface of a back side transparent electrode in the solar cell so that light arriving at the back side is reflected at the interface between the back side transparent electrode and the back side metal electrode, and completing the reflection within a cell to confine the light. When a back side metal electrode is formed over the entire back surface as described above, the use amount of a back side metal electrode material tends to increase, leading to an increase in cost. Accordingly, it is desired to reduce the material cost of the back side metal electrode.
In mass production of solar cells, a plurality of solar cells include good products and defective produces. Conventionally, good products and defective products are judged by applying a current to measure solar cell characteristics (current-voltage characteristics (also referred to as I-V characteristics)) using solar cells after formation of a collecting electrode on the light-receiving side and a back side metal electrode on the back side. For example, Patent Document 1 discloses a method for measuring the electrical characteristics of a solar cell after formation of a collecting electrode on the light-receiving side and a back side metal electrode.
Patent Document 2 discloses bonding a metal plate or a metal foil to a back side collecting electrode (Ag paste) or a back side transparent electrode of a solar cell with an electroconductive adhesive containing Ag fine particles etc. Patent Document 2 mentions that damage by an external force during transportation, stress in the process for sealing a filler or the like as in conventional processes can be suppressed with the above mentioned structure, when a tab line and back side collecting electrode are joined together to modularize the solar cell.